The present invention relates to an electron beam device having a focusing lens. In the present specification the term electron beam device is to be understood to include cathode ray tubes, X-ray tubes, electron beam lithography apparatus, scanning and transmission electron microscopes, electron guns for scanning auger mass spectrometers and also ion guns (not an electron beam device within the normal meaning of the term). For convenience of description, the electron beam device will be described with reference to a cathode ray tube.
Known types of focusing lenses for cathode ray tubes, for example display tubes, are electrostatic bipotential and unipotential lenses, combinations thereof and magnetic lenses. In general, the spherical aberration of lenses decreases with increasing lens diameter.
In the case of electrostatic lenses the maximum diameter is limited by the diameter of the tube neck. However, this restriction does not apply to magnetic lenses, but these are unattractive anyway because of their high power dissipation, their extra weight, the rotation of the electron beam and alignment problems.
It is known, for example from U.S. Pat. No. 4,370,594, that spherical aberration can be reduced by using an electron lens having a long focal length. This specification describes an embodiment of a bipotential lens having two spaced apart cylindrical lens electrodes carried by glass rods in the customary manner. Between the lens electrodes is provided a resistive stack comprising a plurality of plates electrically insulated from each other by means of blocks of an electrically insulating material. A resistive layer bridges the insulating blocks so that a small current can flow therethrough to enable an electric field to be set-up.
U.S. Pat. No. 3,995,194 discloses another electron gun having an extended field focusing lens comprising at least three, and preferably four, discrete focusing electrodes at different voltages which establish a single, continuous electrostatic focusing field during tube operation which field decreases smoothly and monotonically from an intermediate relative potential to a relatively low potential and then increases smoothly, directly and monotonically from the relatively low potential to a relatively high potential. An electron lens disclosed in U.S. Pat. No. 4,124,810 seeks to improve on this prior electron gun by having a distributed electron lens constituted by three electrodes which are at progressively higher voltages in the path of movement of the electron beam from the electron gun to the screen. It is said that a smaller electron spot than that obtained with the previously described electron gun (U.S. Pat. No. 3,995,194) is achieved, if the length of the intermediate electrode of the three electrodes is substantially equal to the lens radius and preferably the voltage change across the intermediate electrode of the three electrodes increases monotonically along the beam path and closely approximates an exponential curve.
All these known lenses require the precision assembly of discrete electrodes which are spatially positioned relative to each other by glass rods. In many cases, each of the electrodes requires a separate voltage supply which in turn means a respective external connection. As the trend in display tube manufacture is towards narrower necks then the size of the electron guns becomes smaller leading to increase of the spherical aberration. Consequently the use of discrete electrodes having their own external connection mitigates such a trend.
In the case of single gun tubes used for monochromatic display there have been proposals for helical electrostatic electron lenses formed by providing conductive helices either directly on the interior of the tube envelope or on the interior of a tubular element of an electrically insulating material, which element forms part of the electron gun. U.S. Pat. No. 3,143,681 discloses that it can be shown mathematically that focusing of an electron beam having axial symmetry can be obtained with a minimum of spherical aberration by an electrostatic field having equipotential surfaces which are co-asymptotic hyberboloids of revolution rotationally symmetrical about the beam axis. A field having the desired hyperboidal equipotential surfaces can be produced by a single electrode consisting of a continuous helical conductor disposed coaxially with a reference axis which may be the longitudinal axis of a cathode ray tube, and having a physical configuration and electrical resistance characteristics such as to produce a space potential at the reference axis which potential varies as a quadratic function of displacement along the reference axis. The specification discloses that the variation in voltage along the helical conductor can be provided by for example varying the effective resistivity of the helical conductor, varying its cross-sectional dimensions, varying its pitch, varying the proportion of turn width to turn spacing, or varying two or more of the foregoing factors in combination to provide a non-linear or non-uniform conductor. Additionally the citation suggests that the desired voltage variation may be achieved by a series of stepped helices, each step or increment being in itself linear but the aggregate having an overall non-linear effect, much as a curve can be approximated by a series of straight lines. However in order to fulfill the required space potential on the electron gun axis it is desirable that the or each helix be terminated by a physical field boundary element having a shape corresponding substantially to the contour of the desired adjacent field equipotential. Such field boundary elements, which may comprise plates or meshes, may as a result of electron impingement form local sources of heat. Such plates and meshes are relatively difficult to design and fabricate and therefore constitute an extra cost item. The presence of such plates and meshes are also undesirable in electron beam devices because they intercept part of the beam current leading to a loss of brightness.
In spite of these proposals no satisfactory general solution exists for designing focusing lenses having a low spherical aberration, which lenses can be used in narrow necked display tubes such as projection television tubes.